1. Field of the Invention
The present invention relates to a hot melt ink thermal transfer recording sheet and a process for producing same. Particularly, the present invention relates to a hot melt ink thermal transfer recording sheet useful for recording thereon clear dotted ink images having a satisfactory color density, and enhanced continuous color tone reproducibility and dot reproducibility when subjected to a hot melt ink thermal transfer printer using a thermal head, and a process for producing same.
2. Description of the Related Art
It is known that a hot melt ink thermal transfer recording system equipped with a thermal transfer ink sheet and a thermal head has a simple mechanism and can be easily maintained and thus is widely utilized as a printer for word processors and facsimile machines. Usually, as a hot melt ink thermal transfer recording (image-receiving) sheet for the system, a fine paper sheet is utilized.
Recently, a thermal transfer full color image recording system was developed, and thus, to enhance the continuous color tone reproducibility, the conventional printer in which a continuous color tone is obtained without changing the size of the individual dots, was changed to a new type of printer in which the continuous color tone is obtained by changing the size of the individual dots. Also, the hot melt ink thermal transfer recording sheet for full color image-recording system in range of applications from low energy to high energy is required to have good recording qualities including an excellent dot reproducibility at which the dot forms of thermally transferred hot melt ink are faithfully recorded, and a high color density for which a sufficient amount of the hot melt ink must be transferred.
Also, since full colored images or pictures are required to be thermally transferred, the recording sheets for the full colored images must accommodate the requirement. When a conventional non-coated printing paper sheet is used for the hot melt ink thermal transfer full colored image-recording system, it often occurs that the color density of the recorded images decreases probably due to a low heat-insulating property of the paper sheet, and the dot reproducibility decreases probably due to a poor cushioning property of the paper sheet. Also, when the surface of the recording sheet is too rough, the resultant recorded images are unclear because of frequent occurrence of missing and/or partial ink dots. Further, when the recording sheet surface is too smooth, the printed ink images are not sufficiently anchored or fixed to the recording sheet surface, and returns back to the hot melt thermal transfer sheet, and thus the resultant recorded images are defective and unclear. The above-mentioned phenomena causes a decrease in the dot reproducibility. Beside the increase in the color density of the recorded images due to the low dot reproducibility, sometimes a decrease in color density of the recorded images occurs due to a low absorption of the ink by the hot melt ink-receiving layer.
Many attempts have been made to solve the above-mentioned problems. Japanese Unexamined Patent Publication Nos. 2-89,690 and 64-27,996 disclose an undercoat layer formed on a substrate sheet and containing hollow particles to enhance the cushioning property of the recording sheet. However, the resultant recording sheets of the prior arts are still unsatisfactory in the cushioning and heat-insulating effects. Also, when the hollow particles are soluble in an organic solvent of a coating liquid for the hot melt ink-receiving layer, it becomes necessary that the hollow particles are bonded by a specific binder consisting of a polymeric material resistant to the organic solvent or an overcoat layer comprising the polymeric material resistant to the organic solvent is formed on the hollow particle-containing ink-receiving layer. The necessity causes the resultant recording sheet to be complicated in constitution. In another attempt for solving the problems, Japanese Unexamined Patent Publication No. 2-41,287 discloses an ink image-recording sheet having an enhanced ink-receiving capacity and produced by forming a resin coating layer containing a component soluble in water on a substrate sheet comprising as a principal component, a plastic resin; and removing the water-soluble component from the resin coating layer by extraction. However, the resultant ink image-recording sheet is disadvantageous in that the highest color density of the recorded images is insufficient or the received ink images exhibit an insufficient gloss. Therefore this type hot melt ink image-recording sheet cannot fully meet the requirements. Also, this type of recording sheet is further disadvantageous in that since the substrate sheet comprises, as a principal component, a plastic resin, the recording sheet is difficult to recycle for reuse.